Paper with high quality finish may be created by applying a thin layer of coating material to one or both sides of the paper. The coating is typically a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder of the organic type or of a synthetic composition. Rosin, gelatins, glues, starches or waxes may also be applied to paper for sizing.
Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers and other applications requiring good printing characteristics for color photos or other specialized paper qualities.
Various devices have been employed in the past to apply coatings to paper, either directly, by flooding the web as it passes through a pond, as in a short dwell coater, or by first applying the coating to a roll, as in a size press. Once coating has been applied to the substrate, it is necessary to meter the coating to a desired thickness and uniform level. Uneven coating thickness will produce blemishes and quality variances in the finished paper, and is highly undesirable.
The pond of coating material employed in the short dwell coater is formed by feeding an excess amount of coating material into a pond housing positioned beneath a backing roll over which a paper web is wrapped. The pond is caused to overflow in the up machine direction, thereby flooding the web and pre-wetting it as it approaches the pond. Downstream from the pond, a metering element, such as a blade, controls the amount of coating material that is applied to the web. The excess coating metered by the metering element is turned downwardly into the pond creating a recirculating zone between the down machine end of the pond and the coating feed at the up machine end of the pond where the excess coating overflows.
The trend in papermaking is to increase efficiency by increasing the speed of formation of the paper. Coating costs can be minimized by coating the paper while still on the papermaking machine. Increasing the paper web speed is critical to continued increases in papermaking productivity. But, because the paper is made at higher and higher speeds, and because of the advantages of on-machine coating, the coaters in turn must also run at high speeds. The need to produce lightweight coated paper to hold down the weight of the paper, and the costs of the coating material, makes the use of short dwell coaters more desirable, since, by subjecting the paper web to the coating material for a short period of time, the depth of penetration of the coating is limited, resulting in a lower coating weight.
However, the use of short dwell coaters at high machine speeds can lead to defects in the coating, typically coating streaks. As the speed of the machine increases, the fluid flow in the pond becomes chaotic and the recirculating flow forms a vortex. The result of the turbulent, chaotic flow is that the location where the paper becomes wetted by the coating begins to oscillate so uniformity of coating contact in the machine direction and the cross machine direction is lost. The turbulent flow which causes streaking in the coating is responsive to adjustments in the coater. Nonetheless, as machine speeds increase, greater control of coater parameters is required.
What is needed is a metering device which can be adjusted to overcome the problem of streaking caused by higher operating speeds.